The roof of many large crude storage tanks is open in the sense that there in no permanently attached roof. The roof floats on top of the product. To prevent vapors from escaping from around the edge of the roof, it is common to provide some sort of seal. These seals are made of a non-conductive material, usually neoprene. This material isolates the roof from the tank wall electrically and from any connection to earth. To overcome this problem, the industry has installed a device called a “shunt.” These shunts are attached to the roof in such a manner that they are to be in constant contact with the tank regardless of the position of the floating roof. To make contact, these shunts are made with metal fingers which are spring-loaded and are made springy by the material used. These shunts require constant maintenance for several reasons:
1. Since the roof floats, it can easily drift slightly off-center and disconnect from the shunt on the opposite side.
2. Wax and other heavy crude elements tend to deposit between the tank wall and the shunt fingers, leaving a small gap which can cause arcing and fire.
Such a fire is called a “rim fire”. In this case, the arc ignites the gasses always prevalent around the rim just above the seal.
During an electrical storm, the electrostatic field will induce a charge on both the tank and the contained product. Lightning dissipaters will discharge both the tank and the product for most situations. However, if the tank has a large diameter, the storm cell contains an unusually large charge, the product near the center will not be completely discharged. If the shunts are not in perfect contact with the tank wall, the “bound charge” will create an arc when that storm cell is discharged by a nearby strike. Refer to the American Petroleum Institute (API) Recommended Practices RP2003 for details on the Bound Charge/Secondary Arc. Some companies have tried to use long wires that extend from the top of the tank down to the center of the floating roof. The problem with this technique is that the impedance of the wire is far too high to react within the time available to discharge a bound charge (about one microsecond. The average impedance of these connections is between 150 and 250 Ohms at lightning frequencies.
Eliminating the risk of a bound charge arcing requires making a full-time positive connection between the tank wall and roof of the tank, as well as having a low impedance path of about one Ohm or less. Making a positive connection will eliminate this risk and any other phenomena that can create a body of bound charge. The present invention discloses a grounding system that will satisfy the above requirements in one complete assembly. The copper braid has an impedance of about one Ohm.